1. Field of the Invention
The present invention relates generally to voice actuated recording devices, and more particularly to a voice operated (VOX) recording device in which speech data that might be lost from recording due to time delays in the VOX circuitry and other recording components of the device, is recovered and stored for recording in proper sequence with speech sounds which follow after the VOX circuitry signals the device to record.
2. Description of the Known Art
Recording devices are used to capture various data including speech in analog or digital form. The data is permanently recorded on media such as magnetic tape. Several different methods of recording are possible. First, the recording device can be left in an ON state over a relatively long time during which an event to be recorded may occur. Second, if the operator of the device knows that an event to be recorded is about to happen, for example, someone giving dictation knows that he or she is about to speak, the person can manually start the recording device before speaking by operating a record button and setting the device in an ON state early enough to capture all of the speech. Third, some kind of automatic start and stop feature responsive to the presence of speech, e.g., VOX control, can be embodied in the recording device.
The first mentioned recording method has the obvious disadvantage that the recording device might be set in an operating state over a time substantially longer than that during which any speech is present. Once incident on the device, however, the speech will be captured in its entirety on the recording medium. This method therefore gives rise to a great deal of wasted media if the speech events are only occasional, with less waste as the speech becomes more continuous.
In the second method--manual operation of the recording device--the operator must be sure to set the device in an ON state before commencing speech. If done properly, the recording media will be used much more efficiently in terms of blank or wasted space.
In the third method of recording mentioned above, a sensor arrangement known as voice operated switch or a VOX is used to detect the speech and, once detected, sets the recording device in an ON state by outputting a VOX speech detection signal. The media is used efficiently in that pauses greater than the cut-off time for the speech detection signal are not recorded. Upon playback, the listener need not wait through a number of long pause intervals.
Accordingly, the most efficient use of recording media for dictation and other speech applications entails automated ON-OFF control of the recording device, wherein the device records only upon the presence of speech. VOX circuitry can be used to accomplish such automation. There are, however, problems associated with the conventional VOX actuated recording devices. Specifically, in a typical speech recording device such as a tape recorder, a length of magnetic tape is driven by a mechanical motor. The motor drives the tape up to an operating speed so that proper recording can take place. Using the VOX speech detection signal as a means for starting the motor therefore has the disadvantage that speech present before the speech detection signal is produced will not be recorded at the beginning of the recorded data. Yet, this initial speech includes the beginning sounds of words and usually contains critical phonemes. If this speech is lost from recording, it may be impossible for the listener during playback to comprehend the initial part of the recording. In other words, VOX circuitry must respond to the presence of speech in order to actuate the drive motor, but since the VOX has an inherent finite time delay, and since the drive motor does not come up to speed instantaneously, a time lag between the beginning of speech and the recording of the speech is created. The portion of the speech omitted from recording corresponds to this time lag.
Further, when speech is completed or a relatively long pause is taken between words and sentences, the VOX ceases to produce an output signal and the drive motor is stopped. Again, there is a finite response time for the VOX to cut off its output signal once speech is no longer present. That portion of the record media which continues to be driven over this response time is wasted since no speech is being recorded. Finally, VOX circuitry may switch ON and OFF during a sentence or word, instead of between sentences and pauses. By causing the tape drive to be turned OFF and back ON quickly, some of the speech may be lost from recording.
U.S. Pat. No. 4,893,197 issued Jan. 9, 1990, discloses sound recording apparatus in which pauses during speech to be recorded are compressed into the form of pause length data bytes, so as to minimize waste of recording media. The patented apparatus does not use a VOX arrangement for controlling the speech recording process, but instead remains in an "on" state all the time. An input analog audio signal is converted to digital form and loaded at successive addresses in a temporary store. Pauses of more than a certain length are detected and their lengths are encoded. Data corresponding to the encoded pause length is then loaded in the temporary store at an appropriate address, and additional addresses of the temporary store are not written until an end of each pause is detected. The data structure of the recording apparatus of the '197 patent is therefore comprised of simple speech data bytes and specially encoded pause length data bytes. It will be appreciated that such a data structure requires substantial central processing unit (CPU) resources to create during record, and to decode, upon a playback operation.